In conventional magnetic recording, thermal instabilities of the stored magnetization in the recording media can cause loss of recorded data. To avoid this, media with high magneto-crystalline anisotropy (Ku) are required. However, increasing Ku also increases the coercivity of the media, which can exceed the write field capability of the write head. It is known that the coercivity of the magnetic material of the recording layer on the disk is temperature dependent. Thus one proposed solution to the thermal stability problem is thermally-assisted recording (TAR), also called heat-assisted magnetic recording (HAMR), wherein high-Ku magnetic recording material is heated locally during exposure to the magnetic field from the write head to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature of approximately 15-30° C.). In some proposed TAR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a magnetoresistive read head. TAR disk drives have been proposed for both conventional continuous media, wherein the magnetic recording material is a continuous layer on the disk, and for bit-patterned media (BPM), wherein the magnetic recording material is patterned into discrete data islands or “bits”.
One type of proposed TAR disk drive uses a laser source and an optical waveguide coupled to a near-field transducer (NFT) for heating the recording material on the disk. A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with subwavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a subwavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of the air-bearing slider that also supports the read/write head and rides or “files” above the disk surface.
TAR disk drives with thermal fly-height control (TFC) of the read/write head have also been proposed. The slider has a disk-facing air-bearing surface (ABS) that causes the slider to ride on a cushion or bearing of air generated by rotation of the disk. The separation or spacing between the head and the disk surface is called the fly-height. The slider is attached to a suspension and the suspension includes a load beam that applies a load force to the slider to counteract the air-bearing force while permitting the slider to “pitch” and “roll”. The flying dynamics of the slider and thus the fly-height are influenced by factors such as the rotation speed of the disk, the aerodynamic shape of the slider's ABS, the load force applied to the slider by the suspension, and the pitch and roll torques applied to the slider by the suspension. TAR disk drives may use TFC for changing the spacing between the head and the disk surface. One type of TFC uses an electrically-resistive heater located on the slider near the head. When current is applied to the heater the heater expands and causes the head to expand and thus move closer to the disk surface. The head can be adjusted to different heights, depending on whether the drive is reading or writing. Also, the heater can maintain the head at the optimal fly-height even in the presence of the above-described factors which would otherwise cause changes in the fly-height.
However, even in the absence of heat to the heater, the write head expands during a write operation as a result of heat from the write coil, which results in protrusion of the write pole. Thus the fly-height of the write pole decreases roughly exponentially during a write operation. To compensate for this, the current to the heater can be decreased to increase the fly-height. However, the time for the optimal fly-height to be achieved can vary. Because the laser is on during a write operation so that the NFT can heat the recording layer, if the optimal fly-height is not achieved or cannot be maintained data errors can occur during writing.
What is needed is a method for controlling the writing in a TAR disk drive with TFC.